The invention relates generally to miniature printed circuit boards (PCB) for microelectrical applications. More particularly, the invention relates to multi-layer and stackable miniature printed circuit boards for static electromagnetic components such as transformers and inductors.
Transformers and inductors are widely known electro-magnetic components used in electrical devices and power supply units. In general, static magnetic components such as transformers and inductors have traditionally been constructed using windings of ordinary conducting wire having a circular cross section. The conventional transformer comprises an insulator gap between a primary coil and a secondary coil, and the voltage generated in the secondary coil is determined by the voltage applied to the primary coil multiplied by the winding ratio between the primary coil and the secondary coil. Manufacture of these traditional structures involves winding the wire around a core or bobbin structure, a process that often involves considerable amounts of expensive hand labor. Furthermore, high power applications often require a magnetic component having a bulky core and large wire sizes for the windings. Even though transformers and inductors are often essential components of an electrical apparatus, they have been historically the most difficult to miniaturize.
New operational requirements with respect to circuit size and power density and the increasing necessity to reduce circuit manufacturing costs have made the traditional static magnetic component very unattractive as a circuit component. Newly designed circuits, for example, need low profiles to accommodate the decreasing space permitted to power circuits. Attaining these objectives has required the redesign of magnetic components to achieve a low profile and a low cost component assembly.
Planar magnetic components fabricated with flexible circuit and multi-layer printed circuit board (PCB) technologies offer an alternative to address the new operational and cost requirements. With planar technology, transformers have been formed from single or multi-layered printed circuit boards. FIG. 1A illustrates an example of a typical planar transformer constructed from printed circuit boards. Specifically, FIG. 1A depicts a side view of such a component 100 attached to the main board 110 of an electrical device. The component 100 includes a PCB 130 with multiple internal layers. Windings of the PCB 130 are connected to the main board by connecting pins 140 and 150. FIG. 1B illustrates the manner in which the component 100 is assembled and FIG. 2 schematically depicts the individual layers of the PCB 130.
The basic construction of the component 100 comprises a spiral conductor on each layer of the PCB 130 forming one or more inductor xe2x80x9cturns.xe2x80x9d As shown in FIG. 1B, the core 120 can comprise two separate and identical E-shaped sections 122 and 124. Each E-shaped section 122, 124 includes a middle leg 126 and two outer legs 128. A hole 132 is drilled in the center of the PCB 130. The middle leg 126 of the E-shaped section 122, 124 can be supported within the hole 132 to form part of the core 120. The middle leg 126 has a circular cross-section and each of the outer legs 128 has a circular or rectangular cross-section. The remaining section of the E-shaped sections 122, 124 is formed by a ferrite bar, which is bonded to the legs 126, 128. The E-shaped sections 122, 124 are assembled so that the legs 126, 128 of each E-shaped section are bonded together. Primary and secondary pins connecting the primary and secondary windings, respectively, can penetrate the PCB via terminal holes 134 drilled near the outer edges of the PCB as will be explained below.
The width of the spiral conductor depends on the current carrying requirement. That is, the greater the current carrying requirement, the greater the width of the conductor. Typically, a predetermined area is reserved for the inductor and the one or more turns are printed on each layer according to well known printed circuit board technology. (See, for example, U.S. Pat. No. 5,521,573.) After each layer is so printed, the layers are bonded together into a multi-layer PCB by glass epoxy. Through-hole xe2x80x9cviasxe2x80x9d or blind xe2x80x9cviasxe2x80x9d are used to interconnect the turns of the different layers.
A through-hole via is formed by drilling a hole through the layers at a position to intersect ends of two of the spiral conductors and then xe2x80x9cseedingxe2x80x9d the inner surface of the holes with a water soluble adhesive. Next, copper is electrolessly plated on the adhesive to interconnect the conductors. Next, additional copper is electrically plated over the electroless copper plate to the desired thickness. Finally, the holes are filled with solder to protect the copper plate. A separate via is required for each pair of spiral conductors on adjacent layers to connect all of the turns in series. Each such through-hole via is positioned not to intersect the other conductors.
Drilling holes in selected layers before the layers are bonded together forms a xe2x80x9cblindxe2x80x9d via. Then, the layers are successively bonded together and, while exposed, the inner surface of the holes is seeded with nickel, electrolessly plated with copper and then filled with solder. The resultant vias extend between the two layers sought to be electrically connected. Thus, the hole does not pass through other layers, and no area is required on these other layers to clear the via. However, the blind via fabrication process is much more expensive than the through-hole fabrication process. Referring back to FIG. 1A, primary pins 140 connecting the primary windings (not shown) and secondary pins 150 connecting the secondary windings (not shown) are then positioned to penetrate the multi-layer PCB 130.
FIG. 2 illustrates a process for manufacturing a printed coil with conventional planar technology in a PCB. In the layers of the PCB of FIG. 2, a primary winding and secondary winding can be formed by connecting multiple coil traces from five layers 200, 220, 240, 260, and 280. The primary winding, for example, can have an outside terminal 202 connected to a coil trace 204 on layer 200. The inside terminal of the coil trace 204 can be connected to an inside terminal of a connection trace 242 on layer 240 by an inner peripheral terminal 208 through a via. The outside terminal of the connection trace 242 is connected by a primary terminal 210 through a via to an outside terminal 282 of a coil trace 284 on layer 280. The inner terminal of the coil trace 284 is connected to the inner terminal of connection trace 244 on layer 240 by a peripheral terminal 286 through a via. Connection trace 244 is connected to outside terminal 246, thereby forming a primary winding between outside terminals 202 and 246 from coil traces 204 and 284 on layers 200 and 280, respectively.
A secondary winding can be formed by connecting a coil trace 224 on layer 220 and a coil trace 264 on layer 260 in a similar fashion. An outside terminal 262 of coil trace 264 can be connected through a via to a corresponding outside terminal 222 of coil trace 224 by a primary terminal 266. The inside terminal of coil trace 224 is connected to the inside terminal of coil trace 284 through a via by peripheral terminal 226. Because the inside terminal of each coil trace 224 and 264 is connected and the outside terminals of each coil trace 224 and 264 is connected, the coil trace 224 and the coil trace 264 are connected in parallel.
FIG. 3 illustrates a typical twelve-layer layout where each individual layer is shown separately. These layers can be connected in a fashion similar to that described above with reference to FIG. 2 to form a PCB having a primary winding and a secondary winding. In this conventional layout, a twelve layer PCB includes traces of both the primary and secondary windings as similarly described with reference to FIG. 2. However, as a result, the primary and secondary windings are physically positioned near or in actual contact with one another, creating significant risks of electrical flashover.
FIG. 4 schematically illustrates how a primary winding and a secondary winding from a PCB can be arranged as a transformer. Referring again to FIG. 2, the windings traced on the layers of a PCB can form a primary winding with external terminals 202 and 282 and a secondary winding with external terminals 226 and 262. As shown in FIG. 4, a primary winding 420 can be connected to the main board 110 by pins 430 and 440 at terminals 202 and 282. A secondary winding 460 can be connected to the main board 110 by pins 470 and 480 at terminals 226 and 262. The primary winding 420 is configured across from the secondary winding 460 with the dielectric material of the core 120 positioned therebetween and represented by lines 490.
While a considerable improvement over traditional construction of magnetic components, these arrangements still fail to meet the performance and cost objectives of contemporary circuit designs. In particular, this conventional planar arrangement poses significant design, cost, and operational disadvantages.
As discussed above, applications today are increasingly demanding space restrictions for their design. Consequently, efforts are continuing to further reduce the size of electrical components. Power supplies, for example, have been significantly reduced in size over the past few years. As a result, the space available for the planar magnetic component is extremely limited. Therefore, the current twelve layer arrangement in conventional planar technology offers a significant obstacle to miniaturizing circuit designs.
Closely tied to the current and ongoing size constraints are the ever-increasing demands for less expensive and more reliable applications. The conventional twelve-layer planar components also prove to be extremely costly. The conventional planar magnetic component must be customized for each circuit design depending on the parameters required (e.g., the turn ratio). If the parameters change, then a new planar magnetic component must be custom manufactured. Manufacture of the magnetic components using conventional planar technology therefore requires substantial costs associated with each new PCB configuration built for each and every circuit parameter change.
Moreover, the current planar technology raises serious operational problems associated with high potential (HIPOT) applications as well. The pins in the conventional boards penetrate the PCB layers in various locations and generally propagate through the thickness of most or all of the layers; however, only certain pins are electrically bonded to certain layers. Because of the manner in which the pins in the conventional planar components fully penetrate the boards in various locations, with only certain pins electrically bonded to certain layers, significant risks of failure due to an electrical flashover exist. Lastly, such many layer boards require significant pressure to laminate them together, thereby generally creating higher shear forces on the layers during manufacture. The resulting lateral movement of each individual layer relative to the layers above and below can cause significant defects to the operation of the component and, in particular, can infringe the minimum space needed between primary and secondary windings.
Accordingly, there is a need for a static electro-magnetic component which not only satisfies demanding operational and size requirements of current electronic technology but also avoids the flashover problems and high costs of the current planar technology. Furthermore, there is a need for an electrical device which offers the additional benefit of providing a configurable and customizable capability allowing a user to change parameters of the component to suit the needs of a particular application.
The embodiments of the invention described below offer an integrated magnetic component utilizing multi-layer stackable PCBs and combine the storage capability of an inductor with the step up, step down or isolation benefits of a transformer in a single structure for high frequency, high density, direct current to direct current (DCxe2x80x94DC) SMPS converters. The novel arrangement of this invention along with its customizable configuration can overcome the disadvantages and problems associated with the prior art.
One embodiment of the invention includes a plurality of core members and a plurality of printed circuit boards stacked into a multi-layer configuration between the core members. A first printed circuit board is configured to form a primary winding of a transformer. A second set of printed circuit boards is configured to form a secondary winding of a transformer. A conductive plate is configured as an output inductor turns. Connector pins are configured to electrically connect the plurality of printed circuit boards to the main circuit board. Each connector pin penetrates only printed circuit boards containing the primary winding or the printed circuit boards containing the secondary winding.
Another embodiment includes three ferrite core portions. One core portion is used in the transformer and one core portion is used in the inductor, and the transformer and the inductor share the middle core portion. The windings of the transformer and the inductor are connected so that the flux created by the transformer and the flux created by the inductor subtract from each other, thereby minimizing the size of core portion shared by the transformer and inductor.
Another embodiment comprises a method of manufacturing an electrical device including printing at least one coil on each of a plurality of printed circuit boards, configuring electrical connections on the plurality of printed circuit boards to include the coils on the printed circuit boards so as to define a primary winding and a secondary winding. A conductive plate is configured as an output inductor. The printed circuit boards and conductive plate are configured in a stacked arrangement, and the conductive plate, the primary winding on the printed circuit boards and the secondary winding on the printed circuit boards are connected to a main circuit board with connector pins in such a manner that the connector pins connecting the primary winding only penetrate printed circuit boards containing the primary winding and connector pins connecting the secondary winding only penetrate printed circuit boards containing the secondary winding.